It is common for a transport refrigeration unit to be controlled by a manually operated thermostat. A set point temperature for a space served or conditioned by the refrigeration unit is manually selected on a set point temperature selector, and a temperature input on the thermostat receives an input signal in the form of a resistance value in ohms from a resistor temperature detector (RTD) disposed to detect the temperature of the conditioned space. The thermostat compares the set point temperature with the temperature of the conditioned space, which is usually the temperature RA of the air returning to the refrigeration unit from the conditioned space, called return air, to provide a difference temperature .DELTA.T. A control algorithm then selects the operating condition of heat and speed relays in response to the polarity and magnitude of the difference temperature .DELTA.T, which forces the refrigeration unit to run in an operating mode which will maintain the temperature of the conditioned space in a predetermined temperature range adjacent to the selected set point temperature. For example, the operating mode may be selected from operating modes such as high speed cool (HSC), low speed cool (LSC), low speed heat (LSH) and high speed heat (HSH). The "low" and "high" speeds refer to two selectable operating speeds for a refrigerant compressor, which may be driven by an internal combustion engine, such as a diesel engine, or an electric motor. Examples of low and high operating speeds are 1400 and 2200 RPM, respectively.
It is also common to provide a data logger for such a transport refrigeration unit, such as a data management system available from the assignee of the present application identified as DMS-II D/S, which records the set point temperature, the temperature RA of the return air, the temperature DA of the discharge air, and the like. The data logger has a microprocessor controller which enables it to communicate with a personal computer (PC), with a modem associated with power line carrier communications, with satellite communications, and the like.
The refrigeration unit cannot be operated remotely without replacing the manual thermostat with a more costly microprocessor based temperature controller, and the data logger cannot operate the refrigeration unit because it has no control over the relays, control valves, and the like, of the refrigeration unit.
It would be desirable, and it is an object of the present invention to be able to operate a refrigeration unit remotely, with the refrigeration unit being of the type which has a manually operable thermostat and a data logger, without the necessity of replacing the manually operable thermostat.